Yes, it seems impossible given that such actions take little or no real preparation. And agree I feel for the victims and their friends and family.
Seems like the authorities had had their eye on the terrorist they shot. But the legal system meant he was released from prison because of his youth.
Interior Minister Karl Nehammer described the dead gunman as an "Islamist terrorist", jailed for 22 months in April 2019 after trying to get to war-torn Syria to join Islamic State (IS) jihadists. The 20-year-old had been released early last December under more lenient terms for young adults.
The British police have said in the past that it is impossible to track all the British people they suspect of being Daesh sympathisers or who have returned from fighting with Daesh in Iraq and Syria.